This invention relates to silver-activated zinc sulfide (ZnS:Ag) phosphors. More particularly, it relates to the synthesis and processing of unpigmented base ZnS:Ag phosphors used to provide the blue luminescent component of phosphor screens in cathode ray tubes.
Blue-emitting ZnS:Ag phosphors are used in the manufacture of cathode ray tubes (CRT) which are widely used in color televisions and computer display monitors. Silver-activated zinc sulfide phosphors are easily handled, chemically stable, and have good emission efficiency. ZnS:Ag CRT phosphors have been prepared in a variety of methods. In one conventional method, zinc sulfide is mixed with a sodium chloride flux and a silver-containing material such as silver chloride. The mixture is then baked in a reducing atmosphere containing a sulfiding species such as carbon disulfide or hydrogen sulfide. After baking, the ZnS:Ag phosphor is allowed to cool and the flux is removed through water washing. Additional finishing steps, e.g., milling, pigmenting, and coating with an organic binder, are employed before the phosphor is used to manufacture cathode ray tubes. The phosphor as it exists before these additional finishing steps is referred to as a xe2x80x9cbasexe2x80x9d phosphor. The phosphor after the additional steps is referred to as a xe2x80x9cfinishedxe2x80x9d phosphor. Unless otherwise indicated, the phosphors described herein are base phosphors.
The physical properties of base phosphor particlesxe2x80x94size, distribution, and morphologyxe2x80x94strongly affect the way the finished phosphor behaves in the processing of CRT screens. The particle size distribution of the phosphor needs to be relatively narrow and the particle morphology should be polyhedral in order to obtain high quality screens. Particles which are too large tend to fall off the screen causing pinholes. Particles which are too small stick to areas where their are not desired leading to color cross-contamination. The desired size of the phosphor particles varies according to the type of cathode ray tube being manufactured. Typically, the desired particle size for television picture tubes is about 9 micrometers (xcexcm).
The conventional method of making ZnS:Aq phosphors is inadequate in that it produces phosphors with less than the desired particle characteristics. In particular, the use of the sodium chloride flux does not promote the formation of polyhedral particles, the particle size distribution tends to be very wide with a large amount of fine particles, and the particles tend to be strongly agglomerated.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a ZnS:Ag,Al phosphor having low particle agglomeration and a substantially polyhedral morphology.
It is another object of the invention to provide a method of making a ZnS:Ag,Al phosphor having low particle agglomeration and a substantially polyhedral morphology.
In accordance with one object of the invention, there is provided a blue-emitting ZnS:Ag,Al phosphor powder comprising ZnS:Ag,Al phosphor particles having a substantially polyhedral morphology and a % xcex94D50 of less than about 20%.
In accordance with another object of the invention, there is provided a method of making a ZnS:Ag,Al phosphor which comprises combining an amount of a zinc sulfide source, an amount of a silver source, an amount of an aluminum source, an amount of a mixed flux, and optionally, an amount of an ammonium chloride source, the mixed flux comprising a mixture of sodium chloride and an alkaline-earth metal chloride selected from barium chloride, calcium chloride, magnesium chloride, strontium chloride, and their hydrated salts. The combined sources and mixed flux are fired in a sulfur-containing atmosphere for a time and a temperature sufficient to form the ZnS:Ag,Al phosphor and then the phosphor is washed to remove residual flux and unreacted silver.
In one aspect of the method of this invention, the alkaline-earth metal chloride is added in an amount from 1% to 10% by weight of the zinc sulfide source and the sodium chloride is added in an amount from 1% to 30% by weight of the zinc sulfide source. In a narrower aspect, the alkaline-earth metal chloride is added in an amount from 2% to 6% by weight of the zinc sulfide source and the sodium chloride is added in an amount from 15% to 25% by weight of the zinc sulfide source.
In another aspect of the method of this invention, the zinc sulfide source is a chlorinated zinc sulfide and the amount of ammonium chloride is from 0 to 2% by weight of the chlorinated zinc sulfide. In a narrower aspect, the alkaline-earth metal chloride is added in an amount from 1% to 10% by weight of the chlorinated zinc sulfide and the sodium chloride is added in an amount from 1% to 30% by weight of the chlorinated zinc sulfide. In a still narrower aspect, the alkaline-earth metal chloride is added in an amount from 2% to 6% by weight of the chlorinated zinc sulfide source and the sodium chloride is added in an amount from 15% to 25% by weight of the chlorinated zinc sulfide source.